Rechargeable magnesium batteries have huge potential for applications in large scale energy storage systems due to their low cost and abundant sources. Nevertheless, not much attention has been paid to the development of alternative anodes for magnesium ion batteries (MIBs). Herein, we demonstrate a scalable strategy to fabricate bismuth (Bi)-tin (Sn) biphase anodes with a porous (P) structure and controllable compositions/sizes, through the design of triphase precursors with immiscible elements (with a positive enthalpy of mixing between elements) and selective phase corrosion. Here, one phase was selected as the sacrificial component to form three-dimensional porous channels, which differs from the mechanism by which a porous structure is generated in a classical dealloying process. We systematically investigate how the composition and size of P-Bi-Sn anodes affect their Mg storage properties. As an anode for MIBs, the P-Bi3Sn2 electrode exhibits an excellent reversible capacity retention of over 93% for 200 cycles at 1000 mA g-1. Most importantly, the Mg storage mechanism of P-Bi-Sn anodes was unveiled by operando X-ray diffraction.